1. Field of the Invention
The present invention relates to a spot welding apparatus in which a spot-weld is made by applying pressure onto both sides of a work through a pair of opposing welding electrodes, and specifically to an apparatus incorporated in a spot welding robot in a manner so as to variably control a welding condition, such as a weld time-interval, a welding current, and a pressure applied to a work through the opposing welding electrodes.
2. Description of the Prior Art
As is generally known, a spot welding apparatus is used to produce a spot weld for a work such as panel materials. The spot ks welding apparatus traditionally includes a C-shaped or X-shaped gun arm having a pair of welding tips on its opposing ends spaced apart from each other. The welding tips serve as welding electrodes. One of the welding tips is movable toward or away from the other by way of air pressure produced by an air cylinder which is mechanically linked to the movable welding tip. In actual, use the pressure acting on the work is determined depending on the magnitude of thrust caused by the air cylinder. In recent years, there has been developed a spot-welding robot with a spot welding apparatus. In the event that a spot welding is made onto a work such as panel materials by utilizing the spot welding robot, spot welding would be in general made with respect to a plurality of weld points on one work. To obtain optimal welding, a welding condition for the respective weld point must be often varied. For instance, in case that a spot welding is made onto a work having at least two different plate thicknesses or onto a coated work by using one set of prior art spot welding apparatus with an air cylinder, pressure applied to the work is actually changed by varying the output pressure produced by the air cylinder by means of a pressure control valve such as an electromagnetic solenoid valve or the like. In such a conventional spot welding apparatus with an air cylinder, the relation between the welding time and the welding current could be electronically controlled through a timing control. On the other hand, a pressure which is applied onto the work by the opposing welding electrodes is remained constant, while a spot-weld is being made to a certain weld point. Although the pressure applied onto the work can be changed at each of different weld points, it may be difficult to timely and arbitrarily vary the pressure applied onto one weld point by way of the air cylinder during welding onto the same. Assuming that the pressure applied onto the one work must be frequently changed by means of the air cylinder, the cycle time of the entire welding work may be increased since the air cylinder has a relatively low following characteristics, namely a low responsiveness in a change of the cylinder stroke. In prior art spot welding apparatus with an air cylinder, a welding condition would be limited owing to low control response characteristics of the air cylinder and undesired dispersion of pressure output from the air cylinder. This results in relatively great spatter loss, or unsatisfactory quality of a weld zone.